1. Field of the Invention
The present system relates generally to X-ray tubes and more particularly relates to method and apparatus for biasing of an X-ray tube to control the flow of electrons between a cathode and an anode.
X-rays are used in a variety of examining procedures in diagnostic medicine. In the more common procedures, an X-ray tube emits a beam of X-rays which is passed through a patient. After passing through the patient the X-rays impinge upon a sheet of X-ray film or upon a fluoroscopic system positioned near the patient and opposite the tube. Absorption of X-rays by the patient changes the intensity of the beam. The beam of changed intensity produces a shadow image indicative of the condition of the internal structure of the patient and is an aid in diagnosing illnesses.
For the more common diagnostic procedures, the value of the intensity of the X-rays which are directed to the subject depends upon several factors. An important factor is the range of intensities which will clearly highlight the internal structure of the subject, while minimizing the radiation dosage the patient receives.
Other intensity determining factors relate to the type of recording or monitoring equipment which is used for producing diagnostic information. For example, higher intensity X-rays are required to expose the sheet of radiographic film than are required by the fluoroscopic system for producing a visual image. More specifically, radiography requires on the order of 600 milliamperes of tube current for 10-100 milliseconds, while fluoroscopy requires substantially continuous tube current, but only 7-10 milliamperes or less. The production of radiographs is concerned with exposures of short duration in which images of high resolution are essential. Even though attempts have been made at reducing the amount of energy required for radiographs, such as by using intensifying screens adjacent the radiographic film, relatively high intensities are still required.
Fluoroscopy is advantageous in applications in which the changes in radiopacities in the subject are in motion or are otherwise time varying. For example, an examiner can fluoroscopically monitor the progress of a radiopaque material through the digestive tract, or can observe the diffusion of the material through the circulatory system.
In modern systems image intensifier tubes are used to produce so-called bright fluoroscopy. The intensity of the X-ray beam which impinges upon the image tube must be coordinated with the intended usage of the output from the tube. Some modern examination systems have the combined ability to take photographs and to provide television images. The advantages of such a dual function system are that the examiner can operate substantially continuously in the fluoroscopy mode to observe generally variations in the radiopacity of portions of the subject body, and can make a permanent record in the form of still or "spot" photographs or cine films of the image intensifier output at desired times.
The described system focuses a television camera on an output screen of the image tube for visually displaying the complete examination. The system also provides a cine camera or a spot camera focused on the tube's output screen for additionally recording photographic images at selected times during an examination.
The television camera requires a less intense image on the output screen than does either a cine camera or a spot camera. For example, the television camera requires only 7-10 milliamperes or less while a camera requires up to 30 milliamperes.
In systems such as that described in the SYSTEM patent, the intensity of the output screen of the fluoroscopic device must be increased very rapidly to a relatively high level during the time the photographic camera is being operated and must be decreased very rapidly to the operating level of the television camera for the rest of the time. Since rapid changes in the intensity on the output screen are required, rapid changes in the intensities of the X-ray beam impinging upon the image tube are needed. Additionally, because it is desirable to pass only the minimumly acceptable amount of radiation through the subject, the increased level of intensity required for cine and photographs should be for as short a period as possible.